I hear you, and yet, I have a different perspective on it. I see no possible choice other than the current course Senator Sanders has taken. A 3rd party is not a viable option to gain the presidency, and especially not for someone who self proclaimed to be a socialist. Personally, I am proud to consider myself a socialist as well, although for expediency, I too am registered as a democrat. Back in the day, I was centrist, and only today in this perverted 21st Century America are my views considered far, far, faaar left.
Here's my perspective: I live in a society. (Best one out there compared to the others? No, not any more although we were once. But top 5 now for sure.) And I realize that I have a lot more on the ball than most. By the generosities of the fate of my birth to an American couple, and of being around at the tail end of the baby boom, I would say I have prospered, although not to the extent of those who came before. I have a technical degree from a top 50 publicly funded University and I work hard. Thus, I don't have a problem with "From me, according to my abilities, to others according to their needs." The needs of others like food, clothing, shelter come first, and these require jobs to pay for. The top 1% and their middle managers have been holding down raises because they're off-shoring and outsourcing labor rather than having specific in-house expertise, but I digress. If someone has to work 2 jobs and still can't afford the cost of living to have a family, they will steal from us when we are at work. So we need jobs, they need to pay living wages, and everyone needs to make about the same more-or-less after taxes for us to be one people.
Otherwise we're us and them, with gated communities and separate sets of laws and outlooks, and experiences with core principles like liberty and justice. One class calls for war and another class fights it. And the cops enforce the rules, and the rules are currently being made by the wealthy for the wealthy to get wealthier. We need someone who will take back, through appropriate taxation on Wall Street implements and employees, what belongs to the folks making those transactions.
So what is the fair way to get: From each according to his abilities? It isn't through volunteer work! It's through taxes. And the current system has folks who basically disproportionately compensate themselves for their days labor and then use their ill-gotten largess to pour back into the system so they don't have to pay the progressive rates the rest of us wage-slaves do! Bernie understands this, and has underwritten/co-written The Progressive Budget. I urge you to read it. A trillion dollars in 10 years from the wealthy by taxing the things they (Would that we can only get there! But don't give up trying. For THAT is the TRUE Spirit of being American! Just don't run over anyone else also on the road to get yours, eh?!!) buy, and restoring the estate tax to eliminate aristocracies. Another Bush or Clinton anyone? Jeez, that's so LAME!
Finally, the two party system has again and again, throughout history, been pulled back and forth by fringe wings of either party. Look at the "Tea Party" Their motto taxed enough already is meaning for the common worker, not for the super 1%, although they have usurped that group totally with their: 'don't tax nobody' intellect. However when things get that far to the right and those folks get in office, you get financial collapses, bubbles and hardship for us, and lowered tax rates or refunds, and reduced services for SOCIETY. So I judge his deal with the democrats, to caucus with them when he split from them to become an independent for their abandoning his; (what ... at that time15 year long) historically consistent position, to be shrewd. Totally the thing of legend. Now when he has the opportunity to run for president, he comes back in and takes the party back to where they were in the Clinton, as opposed to Roosevelt, or Johnson days. That is the only way to get in.
Any other alternative, and I only have over half a century of direct experience, plus what I've actually read in real history books, will be an extension of this dark age of corporate control. Blade Runner was a movie. For entertainment people! It wasn't supposed to represent a model for a happy world and a happy life!